Process for depolymerizing nylon-containing waste to form caprolactam

ABSTRACT

The present invention provides an efficient process for the recovery of caprolactam from polycaprolactam-containing waste material. The present process for depolymerizing multi-component waste material comprising polycaprolactam and non-polycaprolactam components to form caprolactam comprises the step of: in the absence of added catalyst, contacting the multi-component waste material with superheated steam at a temperature of about 250° C. to about 400° C. and at a pressure within the range of about 1 atm to about 100 atm and substantially less than the saturated vapor pressure of water at the temperature wherein a caprolactam- containing vapor stream is formed. 
     The formed caprolactam may then be used in the production of engineered resins and fibers.

BACKGROUND OF THE INVENTION

The present invention relates to a process for the depolymerization ofnylon-containing waste to form caprolactam.

Recovery of caprolactam from nylon 6 scrap (in other words, nylon 6polymer that is substantially free of non-nylon 6 materials) has beenpracticed for at least twenty years. In general, nylon 6 isdepolymerized by heating at elevated temperatures, usually in thepresence of a catalyst and/or steam. See U.S. Pat. Nos. 4,107,160;5,233,037; 5,294,707; 5,359,062; 5,360,905; 5,468,900; and Example 5 ofEuropean Patent Application 608,454. The caprolactam produced may beremoved as a vapor stream as taught by AlliedSignal's U.S. Pat. No.3,182,055. An extensive review of the field has been given by L. A.Dmitrieva et al, Fibre Chemistry, Vol. 17, No. 4, March 1986, pp.229-241. Also, see U.S. Pat. No. 3,939,153.

In contrast to the depolymerization of nylon 6, nylon 66, which issubstantially free of non-nylon 66 materials, is depolymerized byhydrolysis as taught by U.S. Pat. Nos. 4,578,510; 4,605,762; and4,620,032.

U.S. Pat. No. 5,266,694 teaches that a mixture of nylon 6 and nylon 66may be depolymerized by use of a catalyst. U.S. Pat. No. 5,310,905teaches that a mixture of nylon 6 and nylon 66 is first separated fromconsumer waste, e.g. used carpet or carpet scrap, by extraction withaliphatic carboxylic acid; the filtrate comprising the acid andextracted nylon 6 and nylon 66 is then depolymerized. U.S. Pat. No.5,241,066 teaches that a mixture of nylon 6 and PET, which is acidinsoluble, is mixed with acid so that the dissolved nylon 6 may beremoved from PET; the removed nylon is then depolymerized.AlliedSignal's U.S. Pat. No. 3,317,519 teaches that a yarn blend ofnylon 6 and PET may be depolymerized by heating with aqueous alkalimetal hydroxide at elevated pressure.

However, in the case of multi-component mixtures or composites thatcontain nylon 6 as one component, recovery of caprolactam is complicatedby the presence of the other components. These other components and/ortheir decomposition products generated under conventional nylon 6depolymerization conditions interfere with the isolation of caprolactamof adequate purity, thus necessitating expensive additional purificationsteps.

It would be particularly beneficial if an inexpensive method could bedeveloped for the recovery of caprolactam from multi-componentcomposites or materials that include nylon 6, such as carpets. Theprospect of recycling such material presents a tremendous opportunity toreduce landfill usage and the costs of disposal, as well as anopportunity to reuse natural resources.

Carpets include a face fiber that is adhered to a backing (support)material which may include jute, polypropylene, latex (such as astyrene-butadiene rubber (SBR)) and a variety of inorganic materialssuch as calcium carbonate, clay, or hydrated alumina fillers. Nylon 6 isoften used for the face fiber. Typically, carpet comprises about 20-55percent by weight face fiber and 45-80 percent by weight backingmaterials. In addition, the fiber contains dyes, soil repellents,stabilizers, and other compounds added during fiber and/or carpetmanufacture. Waste carpet may also contain a host of other impurities,which will collectively be referred to herein as "dirt".

These non-nylon 6 components interfere with caprolactam recovery. Forexample, one of the most difficult problems is that alkaline components,such as the calcium carbonate filler, neutralize acidic catalysts, suchas phosphoric acid, that are conventionally used to promote nylon 6depolymerization, thus requiring the use of increased amounts ofcatalyst. Another problem is that polypropylene and latex partiallydecompose to a mixture of hydrocarbons that co-distill with caprolactam.The remaining, partially decomposed, non-distilled portion, along withthe filler and other inorganic components, renders the reaction mixturevery viscous and difficult to process in conventional equipment.

Indicative of the difficulties encountered in attempting to recovercaprolactam from nylon 6 carpet are the results described in U.S. Pat.No. 5,169,870 (Corbin et al.) and WO 94/06763 (Corbin et al.). InExample 1 of each publication, the crude yield of caprolactam wasreported as 56% from a feedstock obtained by mechanically separating aportion of the carpet backing and subjecting the enriched nylon 6 todepolymerization; steam and 85% phosphoric acid were used in thedepolymerization respectively at the rate of 33 and 0.55 parts per partof crude caprolactam produced. In Example 3 of each publication, acarpet was depolymerized without prior mechanical separation of thebacking; steam and 85% phosphoric acid were used respectively at therate of 51 and 0.30 parts per part of crude caprolactam produced. (Theyield of caprolactam was not stated.) It is evident that the highexpenditure of steam and phosphoric acid, and the low yield ofcaprolactam, render this process economically unattractive. Examples 4and 5 of WO 94/06763 report higher yields of caprolactam, but initialseparation techniques to reduce the amount of CaCO₃ prior todepolymerization were required. U.S. Pat. No. 5,455,346 describes aprocess applicable to the recovery of caprolactam from mixturescontaining nylon 6, including nylon 6 carpets. Initial separationtechniques are also used to increase the nylon 6 content of the mixtureprior to depolymerization; Example 13 teaches that the carpeting wasfreed from polyamide-free components until the polycaprolactam was 75percent by weight based on the mixture. In contrast, it is oftendesirable to avoid such separation techniques.

One way to circumvent the problems associated with the presence ofnon-nylon 6 components in a material that includes both nylon 6 andnon-nylon 6 components involves heating the waste material underpressure in water, separating the resulting solution from the non-nylon6 components, and recovering caprolactam from the aqueous solution byfurther treatment. Processes based on these general principles aredescribed in Czechoslovakian Patent No. 143,502 to Petru et al. and inAlliedSignal's U.S. Pat. No. 5,457,197 to Sifniades et al. Althoughthese processes are an improvement, they suffer from the disadvantage ofrequiring multiple steps and/or high pressure operations with associatedhigher capital investment and operating expenses.

A need still exists, therefore, for an efficient process for recovery ofcaprolactam from multi-component materials that include nylon 6.

SUMMARY OF THE INVENTION

The invention provides another process for depolymerizingmulti-component waste material comprising polycaprolactam andnon-polycaprolactam components to form caprolactam which avoids theproblems associated with the previous recovery methods. The processcomprises the step of: in the absence of added catalyst, contacting themulti-component waste material with superheated steam at a temperatureof about 250° C. to about 400° C. and at a pressure within the range ofabout 1 atm to about 100 atm and substantially less than the saturatedvapor pressure of water at the temperature wherein acaprolactam-containing vapor stream is formed.

Optionally, the multi-component waste material is contacted for a shorttime period with liquid water under elevated temperatures and pressuresprior to contacting with steam as discussed above.

According to preferred embodiments, the multi-component waste materialis nylon 6 carpet.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in more detail below with reference todrawing, wherein FIG. 1 is a graph illustrating one advantage of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, "multi-component, nylon 6 waste material" denotesmaterial or articles that include nylon 6 and at least one othercomponent which may be a non-hydrolyzable polymer, an inorganic ororganic material, or other types of materials, and that has been, isintended to be, or otherwise would have been discarded by a consumer,manufacturer, distributor, retailer, installer and the like. The othercomponents can constitute from about 5 to about 95, preferably about 20to about 80 weight percent of the multi-component, nylon 6 wastematerial. "Multi-component, nylon 6 waste material" does not includewaste material composed solely of scrap nylon 6 polymeric and/oroligomeric material, such as material generated during the production ofintermediate articles such as fiber, chip, film or molded articles whichintermediate articles are then incorporated or transformed into end usemulti-component products such as carpets and packaging. Examples of suchscrap material are yarn waste, chip waste, or extruder slag.

The multi-component nylon 6 waste material comprises up to a total ofabout 10 percent by weight with respect to polycaprolactam of at leastone of polyhexamethylene adipamide (hereinafter "nylon 66") andpolyethylene terephthalate (hereinafter "PET"). Thus, themulti-component nylon 6 waste material may comprise up to a total ofabout 10 percent by weight with respect to polycaprolactam of nylon 66,PET, or a mixture of nylon 66 and PET. For purposes of convenience,"multi-component, nylon 6 waste material" may be referred to as"multi-component waste material" hereafter. The foregoing weightpercentages exclude the presence of dirt, a previously defined term.

A preferred embodiment is the recovery of caprolactam from waste carpetmaterial that includes nylon 6 face fiber and non-nylon 6 components.

As used herein, "fiber" denotes an elongated body, the length dimensionof which is much greater than the transverse dimensions of width andthickness. Accordingly, "fiber" includes, for example, monofilament,multifilament yarn (continuous or staple), ribbon, strip, staple andother forms of chopped, cut or discontinuous fiber, and the like havingregular or irregular cross-sections. "Fiber" includes a plurality of anyone of the above or a combination of the above.

As used herein, "carpet material" denotes carpet which has not beensubjected to any mechanical separation (referred to herein as "wholecarpet"), as well as any mixture of carpet components that is a productof separation, mechanical or otherwise, of whole carpet (referred toherein as "beneficiated carpet"). "Waste carpet material" denotes carpetmaterial that has been, is intended to be, or otherwise would have beendiscarded by a consumer, manufacturer, distributor, retailer, installerand the like.

According to the process of the current invention, caprolactam is formedby contacting the multi-component waste material with superheated steamat elevated temperatures and atmospheric or higher pressures andremoving a vapor stream containing caprolactam from the contact region.The term "superheated steam" as used herein means steam that is heatedto a temperature substantially higher than the temperature at whichcondensation to liquid water would take place at the pressure used toconvey said steam. An important benefit of the process is that nocatalyst is needed for recovering caprolactam from whole carpet waste.Whole carpet generally includes calcium carbonate, which can neutralizean acidic catalyst.

Accordingly, for depolymerization processes employing an acidiccatalyst, such as phosphoric acid, increased amounts of catalyst arerequired to effect depolymerization, thereby rendering the processimpractical or uneconomical. Accordingly, for the present process, noacidic catalyst is added to the vessel in which the multi-component,nylon 6 waste material is contacted with superheated steam. It should beunderstood, however, that the waste material feedstock may include minoramounts of materials (for example, contaminants) that incidentally arerecognized in the art as catalysts. However, the subject process doesnot rely on the presence or addition of any such catalytic materials inthe vessel.

A further benefit is that even a feedstock composed substantially ofwhole carpet can be employed in the process, with sufficient yields ofcaprolactam. This avoids the need for separation processes, to removevarious components in carpet, prior to depolymerization.

When nylon 66, PET, or a mixture of nylon 66 and PET is present in themulti-component waste material in an amount of up to a total of 10percent by weight with respect to nylon 6, these polymers do notinterfere with the present depolymerization process or subsequentpurification procedures involving distillation of caprolactam. This isan added advantage of the present process, because in carpet recycling,it is virtually certain that small quantities of nylon 66 and PET carpetwill find their way in the nylon 6 carpet feedstock. In contrast, innylon 6 depolymerization processes that rely on liquid phasedepolymerization (see U.S. Pat. Nos. 5,359,062 and 5,455,346), thecaprolactam produced in solution is sensitive to polymerizationinitiated by the adipic and terephthalic acids produced by hydrolysis ofsaid polymers. Therefore, such processes must employ low temperaturemethods, such as extraction, for caprolactam purification, or they mustbe coupled with post-depolymerization procedures, such as inAlliedSignal's U.S. Pat. No. 5,457,197 to Sifniades et al.

The multi-component waste material is preferably fed to the reactor as amelt. This feeding may be achieved by using an extruder, gear pump, orother means known in the art. Some feeding systems, such as extruders,allow the development of relatively high pressures in the melt. Thisoffers the option of contacting the melt with liquid water at elevatedtemperatures for a short period of time at little added cost. This maybe achieved, for example, by introducing water under pressure in theextruder barrel. The contact time between the melt and water may beextended by placing a high pressure pipe between the extruder exit andreactor. In this optional pretreatment step, the multi-component wastematerial is combined with liquid water and heated at a sufficienttemperature for a time period sufficient to effect an initialdepolymerization of the polycaprolactam. The depolymerization productsformed in this step may include reduced molecular weightpolycaprolactam, caprolactam, caprolactam linear oligomers, andcaprolactam cyclic oligomers. Such contact accelerates caprolactamproduction in subsequent process steps as disclosed in AlliedSignal'sU.S. Pat. No. 5,457,197 to Sifniades et al. The disclosure ofAlliedSignal's U.S. Pat. No. 5,457,197 is incorporated herein byreference.

For the recovery of caprolactam to be economical, it is desirable toutilize as inexpensive equipment and as little steam as technicallyfeasible. A good index of the economy of the process is theconcentration of caprolactam obtained in the overheads, which bears aninverse relationships to the amount of steam used. Concentrations inexcess of 15 wt. % can be obtained by appropriate design of the reactorand choice of operating conditions as described below.

The reaction temperature should be at least about 250° C. but not higherthan about 400° C. Generally, the rate of caprolactam formationincreases with increasing temperature. However, the rate of sidereactions of nylon 6 such as evolution of ammonia also increases withtemperature and so does the rate of reactions of the non-nylon 6components of the multi-component material.

Temperatures of at least about 250° C. are preferred because below 250°C., caprolactam formation may be too slow, Temperatures no greater thanabout 400° C. are preferred, as above 400° C. side reactions of nylon 6and reactions of the non-nylon 6 components may become prohibitivelyfast. A preferred temperature range is about 280° C. to about 350° C.,more preferably a temperature in the range of about 300° C. to about340° C.

The pressure should be at least atmospheric but higher pressures offercertain advantages as will be explained below. Other factors, such asthe availability and operating cost of high pressure equipment mayinfluence the choice of pressure.

Regarding the effect of pressure, it has been found that for a giventemperature and steam flow, increasing the reactor pressure generallyincreases the caprolactam concentration in the overheads up to anoptimal pressure. Further small increases in pressure have little effecton caprolactam concentration. However, a large increase in pressurebeyond the optimal pressure results in decreased caprolactamconcentration. Generally, the higher the operating temperature, thehigher is the optimal pressure at which maximum caprolactamconcentration is obtained. For example at about 320° C. and a steam flowof 1 reaction mass per hour, the optimal pressure is about 11 atm (about1114 kPa); at about 340° C. and a steam flow of 2.0 reaction mass perhour, the optimal pressure is about 15 atm (about 1520 kPa). Optimalpressure conditions under different operating conditions within thescope of this invention can be determined by those skilled in the art.

It will be appreciated that the optimal pressure is well below thesaturated vapor pressure of water at the operating temperature. Forexample, the saturated vapor pressure of water is 111 atm at 320° C.,and 144 atm at 340° C. Therefore, it is clear that in the currentprocess, no liquid aqueous phase is present.

The effect of pressure on caprolactam concentration at constant steamflow is matched by its effect on the rate of production of caprolactam.Therefore, operating near the optimal pressure minimizes not only steamusage but also reactor volume.

A further benefit of operating close to the optimal pressure is thesuppression of side reactions leading to ammonia formation. We havefound that at a given temperature, ammonia production duringpolycaprolactam depolymerization is lower the faster caprolactam isremoved from the reaction zone.

Although not wishing to be bound by any theory, it is surmised that aspressure increases at a given temperature and steam flow, the amount ofwater that dissolves in nylon 6 is increased resulting in theacceleration of depolymerization reactions. It will be appreciated thatthe action of water in the depolymerization of nylon 6 to caprolactam iscatalytic, that is, no net amount of water is consumed in the overallconversion of nylon 6 to caprolactam. Caprolactam is generally formed bycleavage of caprolactam molecules from the ends of the nylon 6 chain, ina reversal of the polyaddition reaction which constitutes caprolactampolymerization. Water promotes caprolactam formation by virtue ofpromoting the cleavage of amide bonds, which results in the formation ofmore end groups. Water is consumed only to the extent that some of thenylon 6 charged is not converted to caprolactam. As caprolactam isproduced at a faster rate, its partial pressure in the vapor phaseincreases. However, the partial pressure of water also increases,approximately in proportion to the applied pressure. Thecaprolactam/water ratio in the overheads is proportional to the ratio ofthe corresponding vapor pressures.

Therefore, increasing the reactor pressure can result in an increase ora decrease of caprolactam concentration in the overheads, depending onwhether the caprolactam vapor pressure increases faster or slower thanthe water vapor pressure. Evidently, at pressures below the optimalpressure, the caprolactam partial pressure increases faster than thepartial pressure of water as the reactor pressure is increased. Atpressures above the optimal pressure, the partial pressure of waterincreases faster than the partial pressure of caprolactam as the reactorpressure is increased.

A secondary effect of pressure is the suppression of caprolactam cyclicdimer. The dimer is formed reversibly along with caprolactam duringnylon 6 depolymerization. When the depolymerization is carried out atatmospheric pressure, relatively large amounts of the dimer are found inthe overheads, as much as 3-4 wt % of the caprolactam. Increasing thepressure decreases the ratio of dimer to caprolactam in the overheads.Since dimer formation is reversible, dimer that does not distill over isconverted eventually to caprolactam. Suppressing dimer concentration inthe overheads is beneficial not only from the point of view of productyield, but also because the dimer, when present at high concentrations,may be deposited as a solid and clog the transfer lines and thecondenser.

In view of these findings, the operating pressure should range fromabout 1 atm up to about 100 atm (about 101 kPa to about 10130 kPa).However, the pressure should be substantially less than the saturationvapor pressure of water under the operating temperature to ensure thatliquid water does not condense in the reactor. For example, at 300° C.,the saturated vapor pressure of water is 85 atm. Operation at thattemperature should be carried out at pressures ranging from about 1 atmto about 75 atm. For the preferred temperature range of about 280° C. toabout 350° C., the preferred pressure range is about 1 atm to about 30atm (about 101 kPa to about 3940 kPa). For the more preferredtemperature range of about 290° C. to about 340° C., the preferredpressure range is about 3 atm to about 15 atm (about 304 kPa to about1520 kPa). The rate of steam flow should be sufficient to removecaprolactam from the reactor, but not so high as to cause undue dilutionof caprolactam in the overheads. Since a high caprolactam concentrationin the overheads is desired, the steam flow should be proportional tothe rate of production of caprolactam, which is generally proportionalto the mass of nylon 6 charged and also increases with temperature.

The contact of the multi-component waste material with steam is effectedin a vessel designed to withstand the requisite temperature andpressure, as well as the corrosiveness of the reactants. Since nocorrosive catalysts, such as acids, are required in this process, nospecial alloys are required, and a stainless steel vessel is adequate.

Good contact between steam and the multi-component waste material isessential for an effective operation. Such contact may be achieved byvarious means known generally in the art. As an example, steam may besparged through the material using a multiplicity of inlets, forexample, using a steam distributor. Improved contact may be achieved byincluding mechanical agitation in the reactor, for example, using acombination of rotating paddles and static fins.

The process of the current invention may be carried out eithercontinuously or in batch fashion. In the latter case, themulti-component waste material is charged to the reactor all at once andsteam is sparged continuously until most of the caprolactam has beenrecovered. Generally, in the batch process, caprolactam concentration inthe overheads diminishes as the charge is depleted of nylon 6. Saidconcentration may be maintained at relatively high levels throughout theprocess by gradually increasing the temperature and/or decreasing thesteam flow as the run process.

In a continuous process, both the multi-component waste material and thesteam are fed continuously to the reactor. Caprolactam is recoveredoverhead, while a nylon 6 depleted melt is discharged from the bottoms.To maintain a high caprolactam concentration in the overheads, it isdesirable to run the steam countercurrent to the melt flow. This can beachieved by using a series of continuous stirred reactors (CSTRs) inwhich melt flows from the first reactor to the last while steam flows inthe opposite direction. However, it is also possible to operate withsteam crossflow or crosscurrent flow. In this mode, the melt flows fromthe first reactor to the last, whereas fresh steam is supplied to eachreactor. If desired, the steam flow to each reactor may diminish as thenylon content of the melt diminishes. Although crossflow may generallyresult in higher overall consumption of steam, it is simpler toimplement and may require lower capital investment.

In a preferred embodiment of the process, nylon 6 carpet melt is fed atthe top of a continuous flow reactor. Superheated steam is fed through adistributor at the bottom of the reactor countercurrent to the flow ofthe melt. A vapor stream containing caprolactam is collected at the topof the reactor and nylon 6 depleted melt exits at the bottom. The carpetmay be fed by means of an extruder, gear pump, or other device. Thereactor may be divided into several stages by means of baffles. Meansmay be provided for mechanical agitation in each stage. Heat is providedto the reactor mainly by means of the superheated steam. Additional heatmay also be provided through the carpet feed, especially if an extruderis used to feed the carpet, and through the wall of the reactor.

Caprolactam may be separated from other components of the distillate.The vapors from the reactor overhead may be sent to a partial condenserto obtain a condensate containing caprolactam. Fiber grade caprolactammay be obtained from this condensate by further purification includingdistillation, crystallization and other conventional techniques known inthe art. For example, the caprolactam purification process ofAlliedSignal's U.S. Pat. Nos. 2,813,858; 3,406,176 or 4,767,503 toCrescentini et al. may be used.

The purified caprolactam may then be used to make polycaprolactam usinga known process such as disclosed in AlliedSignal's U.S. Pat. Nos.3,294,756; 3,558,567; or 3,579,483. The polycaprolactam may then be usedin known engineered materials such as disclosed in AlliedSignal's U.S.Pat. Nos. 4,160,790; 4,902,749; or 5,162,440 or spun into fiber using aknown process such as disclosed in AlliedSignal's U.S. Pat. Nos.3,489,832; 3,51 7,41 2; or 3,619,452.

The following examples illustrate various preferred embodiments of theinvention.

EXAMPLE 1

Whole carpet feedstock containing 57.6% by weight nylon 6 was preparedby extruding a shredded carpet having nylon 6 face fiber and backing ofpolypropylene and calcium-filled SBR and grinding the extrudate to 5mesh chips. A 178.8 g portion of the feedstock was placed in acylindrical stainless steel reactor of 24.5 mm diameter and 1070 mmheight. The reactor was connected to a condenser equipped with aback-pressure valve at the exit set at 9.2 atm (932 kPa). Superheatedsteam was blown through the bottom of the reactor at the rate of 3 g/minwhile the temperature of the reactor was maintained at 300° C. Overheadscuts were taken periodically and analyzed for caprolactam, caprolactamoligomers, and ammonia. The concentration of caprolactam reached 15 wt.% by the third cut and gradually declined to 3.8 wt. % as the nyloncontent of the carpet was depleted. Overall, 1094 g of distillate werecollected within 6.0 hours containing 92.5 g caprolactam, 0.54 gcaprolactam cyclic dimer, and 0.126 g ammonia. The molar yield ofcaprolactam based on nylon 6 present in the carpet charged was 89.8%.The moles of cyclic dimer (expressed as caprolactam equivalents) andmoles of ammonia relative to lactam produced were 0.58% and 0.91%respectively.

EXAMPLES 2-7

Several more examples were carried out using the same feedstock andapparatus as in Example 1. In all cases, the charge was 180±2 g. Theresults are summarized in Table 1 below. It is seen that increasing thetemperature at essentially constant pressure and steam rate, the maximumconcentration of caprolactam in the overheads increases (Examples 1 and4); increasing the pressure at constant temperature and steam rateincreases the caprolactam concentration until an optimal level ofpressure is reached, and decreases the yield of caprolactam cyclic dimer(Examples 2-5); and increasing the steam rate at constant temperatureand pressure decreases the caprolactam concentration but increases thecaprolactam yield (Examples 4 and 6). Example 7 shows that highcaprolactam concentration can be achieved at increased steam flow bysimultaneously increasing the temperature and pressure.

The effect of pressure on the rate of caprolactam production isdemonstrated in FIG. 1, in which the cumulative amount of caprolactam inthe overheads is plotted as a function of time for Examples 2-5, inwhich the temperature and the steam flow were held constant at 320 ° C.and 3 g/min respectively. The curves are labeled by the number of theExample to which they refer. It is seen that as the pressure isincreased from atmospheric (Example 2) to 6.1 atm (Example 3), the rateincreases by more than a factor of two. Further increase in pressure to10.9 atm (Example 4) produces a smaller increase in rate. Furtherincreasing the pressure to 14.9 arm (Example 5) results in a smalldecrease in rate, indicating that the optimal pressure under theseconditions of temperature and steam flow is in the range of 11 to 15atm. Additionally, comparing FIG. 1 to Table 1, it is apparent that theammonia to caprolactam ratio bears an inverse relationship to the rateof caprolactam production. Therefore, operating close to the optimalpressure minimizes both dimer and ammonia production relative tocaprolactam.

                                      TABLE 1                                     __________________________________________________________________________    STEAM DEPOLYMERIZATION OF NYLON 6 CARPET                                          Temp.                                                                             Press.                                                                            Steam                                                                             Time                                                                              Maximum                                                                            (a)  (b) (c)                                         Ex. deg C.                                                                            atm g/min                                                                             min Concn %                                                                            Caprol.                                                                            Dimer                                                                             Ammonia                                     __________________________________________________________________________    1   300  9.2                                                                              3   360 15.0 89.8 0.58                                                                              0.91                                        2(d)                                                                              320  1.0                                                                              3   360  5.5 48.1 2.26                                                                              5.03                                        3   320  6.1                                                                              3   360 18.2 91.6 0.82                                                                              1.97                                        4   320 10.9                                                                              3   300 23.4 90.0 0.53                                                                              1.67                                        5   320 14.6                                                                              3   300 22.0 87.2 0.45                                                                              2.29                                        6   320 10.9                                                                              6   300 14.5 95.4 0.68                                                                              1.26                                        7   340 14.6                                                                              6   180 24.2 93.1 0.58                                                                              2.15                                        __________________________________________________________________________     (a) Caprolactam overhead, mol % of nylon 6 charged.                           (b) Dimer overhead, mol % of caprolactam overhead.                            (c) Ammonia overhead, mol % of caprolactam overhead.                          (d) The run for Example 2 was discontinued after 360 minutes; caprolactam     was produced at a low rate and the ratio of dimer and ammonia to              caprolactam was higher than in the other runs.                           

EXAMPLE 8

A carpet having nylon 6 face fiber and backing of polypropylene andcalcium-filled SBR contained about 52% by weight nylon 6. The carpet wascut to strips and about 850 g thereof were charged to a 2 liter stirredautoclave via an extruder. Superheated steam was sparged at the bottomof the autoclave at the rate of 20 g/min while a vapor stream containingcaprolactam flowed overhead and was fed to a partial condenser. Acondensate containing up to 80% by weight caprolactam was collected. Thetemperature and the pressure in the autoclave were maintained at 312 °C. and 9.2 atm respectively during the run. After one hour of operation,the yield of caprolactam in the collected overheads was about 50% byweight of the nylon charged. At the end of three hours, the yield wasover 90% based on the nylon 6 in the starting material. The condensatewas filtered through filter-aid to remove a small amount of oils andsuspended waxes and submitted to fractional distillation under vacuum. Afraction containing over 99% caprolactam was obtained. Less than 10% ofthe available caprolactam remained in the distillation bottoms, Thedistilled caprolactam was further purified via crystallization fromwater to yield fiber quality caprolactam.

EXAMPLE 9

The caprolactam from Example 8 is spun into fiber using a known spinningprocess.

EXAMPLE 10

The procedure of Example 8 was repeated, except that nylon 66 chipscorresponding to about 5% by weight to the nylon 6 present in the carpetwere charged to the autoclave along with the carpet. The rate andselectivity of the depolymerization paralleled that of Example 8.Fractional distillation of the collected overheads produced a fractioncontaining over 99% caprolactam, and less than 10% of the availablecaprolactam remained in the distillation bottoms.

EXAMPLE 11

The procedure of Example 10 was repeated, except that polyethyleneterephthalate chips were substituted for the nylon 66 chips. Comparableresults to Example 10 were obtained.

COMPARATIVE EXAMPLE A

One part of a mixture of nylon 6 and nylon 66 chips in the weight ratio95:5 and 6.67 parts of water were placed in a sealed autoclave andheated under autogenous pressure to 300° C. for one hour. Analysis ofthe resulting solution revealed that about 75% of nylon 6 had beenconverted to caprolactam. Because caprolactam polymerizes with themixture of nylon 6 oligomers and nylon 66 oligomers, only a smallportion of the caprolactam can be recovered.

COMPARATIVE EXAMPLE B

The procedure of Comparative Example A was repeated, except thatpolyethylene terephthalate chips were substituted for the nylon 66chips. Comparable results to Comparative Example A were obtained.

EXAMPLE 12

For a continuous process, the apparatus comprises at least threereactors equipped with inlet at the top and outlet at the bottom forliquid flow, and inlet at the bottom and outlet at the top for vaporflow. The three reactors are connected in series so that liquid flowruns in one direction while vapor flow runs in the opposite direction.Each reactor is equipped with a mechanical agitator and baffles thatensure intimate mixing between liquid and vapor. Waste carpet containingabout 50% nylon 6 is shredded and fed to an extruder. The extrudate iscontinuously fed to the first reactor and exits from the last.Superheated steam is fed to the last reactor at a rate approximately 3times the extrudate flow and exits from the first reactor. The reactorsare held at about 330° C. and 12 atm. The overall residence time of themelt in the reactors is about 4 hours. The exit vapors are sent to apartial condenser where a condensate containing about 90% caprolactam isobtained. Fiber grade caprolactam may be obtained from this condensateby further purification including filtration, distillation,crystallization and other conventional techniques known in the art. Aportion of the remaining vapor is purged while the rest is mixed withmakeup steam, sent to a superheater, and recycled through the process.

EXAMPLE 13

The caprolactam from Example 12 is used to make an engineered plastic.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usage's andconditions.

We claim:
 1. A process for depolymerizing multi-component waste materialcomprising polycaprolactam and non-polycaprolactam components to formcaprolactam comprising the step of:in the absence of added catalyst,contacting said multi-component waste material with superheated steam ata temperature of about 250° C. to about 400° C. and at a pressure withinthe range of about 1 atm to about 100 atm and substantially less thanthe saturated vapor pressure of water at said temperature wherein acaprolactam-containing vapor stream is formed.
 2. The process of claim 1wherein said multi-component material comprises up to a total of about10 percent by weight with respect to polycaprolactam of at least one ofpolyhexamethylene adipamide and polyethylene terephthalate.
 3. Theprocess of claim 1 wherein said multi-component material comprisesprimarily waste polycaprolactam carpet.
 4. The process of claim 1wherein said multi-component material comprises at least about 95percent by weight waste polycaprolactam carpet.
 5. The process of claim1 which further comprises the step of: subjecting a mixture of liquidwater and said multi-component waste material to sufficient heat andpressure for a time sufficient to reduce the molecular weight of saidpolycaprolactam prior to said contacting step.
 6. The process of claim 1wherein said pressure is within the range of about 1 atm to about 30atm.
 7. The process of claim 1 wherein said temperature is within therange of about 280° C. to about 350° C. and said pressure is within therange of about 1 atm to about 30 atm.
 8. The process of claim 1 whereinsaid temperature is within the range of about 290° C. to about 340° C.and the pressure is within the range of about 3 atm to about 15 atm. 9.The process of claim 1 wherein the yield of caprolactam in saidcaprolactam-containing vapor stream is at least about 85 percent byweight based on the polycaprolactam content of said multi-componentwaste material.
 10. The process of claim 1 wherein the yield ofcaprolactam in said caprolactam-containing vapor stream is at leastabout 90 percent by weight based on the polycaprolactam content of saidmulti-component waste material.
 11. The process of claim 1 wherein theyield of caprolactam in said caprolactam-containing vapor stream is atleast about 95 percent by weight based on the polycaprolactam content ofsaid multi-component waste material.
 12. The process of claim 1 whichfurther comprises the step of:removing said formedcaprolactam-containing vapor stream from said contact region.
 13. Theprocess of claim 12 which further comprises the step of:separating saidcaprolactam from said removed caprolactam-containing vapor stream bypartial condensation.
 14. The process of claim 13 which furthercomprises the step of:purifying said separated caprolactam.
 15. Aprocess for depolymerizing multi-component waste material comprisingpolycaprolactam and non-polycaprolactam components to form caprolactamcomprising the step of:in the absence of added catalyst, contacting saidmulti-component waste material countercurrently or crosscurrently withsuperheated steam in a series of continuous flow stirred reactors at atemperature of about 250° C. to about 400° C. and at a pressure withinthe range of about 1 atm to about 100 atm and substantially less thanthe saturated vapor pressure of water at said temperature wherein acaprolactam-containing vapor stream is formed.
 16. The process of claim15 wherein said multi-component material comprises up to a total ofabout 10 percent by weight with respect to polycaprolactam of at leastone of polyhexamethylene adipamide and polyethylene terephthalate. 17.The process of claim 15 wherein said multi-component material comprisesprimarily waste polycaprolactam carpet.
 18. The process of claim 17wherein said multi-component material comprises at least about 95percent by weight polycaprolactam carpet.
 19. The process of claim 15which further comprises the step of:subjecting a mixture of liquid waterand said multi-component waste material to sufficient heat and pressurefor a time sufficient to reduce the molecular weight of saidpolycaprolactam prior to said contacting step.
 20. The process of claim15 which further comprises the step of:removing said formedcaprolactam-containing vapor stream from said contact region.
 21. Theprocess of claim 20 which further comprises the step of:separating saidcaprolactam from said removed caprolactam-containing vapor stream bypartial condensation.
 22. The process of claim 20 which furthercomprises the step of:subjecting a portion of said caprolactam depletedvapor stream to superheating and recycling it to said contacting step.23. The process of claim 21 which further comprises the stepof:purifying said separated caprolactam.
 24. A process fordepolymerizing multi-component waste material comprising polycaprolactamand non-polycaprolactam components to form caprolactam comprising thestep of:in the absence of added catalyst, contacting saidmulti-component waste material countercurrently with superheated steamin a vertical tubular reactor at a temperature of about 250° C. to about400° C. and at a pressure within the range of about 1 atm to about 100atm and substantially less than the saturated vapor pressure of water atsaid temperature wherein a caprolactam-containing vapor stream isformed.
 25. The process of claim 24 wherein said multi-componentmaterial comprises up to a total of about 10 percent by weight withrespect to polycaprolactam of at least one of polyhexamethyleneadipamide and polyethylene terephthalate.
 26. The process of claim 24wherein said multi-component material comprises primarily wastepolycaprolactam carpet.
 27. The process of claim 24 wherein saidmulti-component material comprises at least about 95 percent by weightpolycaprolactam carpet.
 28. The process of claim 24 which furthercomprises the step of:subjecting a mixture of liquid water and saidmulti-component waste material to sufficient heat and pressure for atime sufficient to reduce the molecular weight of said polycaprolactamprior to said contacting step.
 29. The process of claim 24 which furthercomprises the step of:removing said formed caprolactam-containing vaporstream from said contact region.
 30. The process of claim 29 whichfurther comprises the step of:separating said caprolactam from saidremoved caprolactam-containing vapor stream by partial condensation. 31.The process of claim 29 which further comprises the step of:subjecting aportion of said caprolactam depleted vapor stream to superheating andrecycling it to said contacting step.
 32. The process of claim 30 whichfurther comprises the step of:purifying said separated caprolactam. 33.A process for depolymerizing multi-component waste material comprisingpolycaprolactam and non-polycaprolactam components to form caprolactamcomprising the step of:in the absence of added catalyst, contacting saidmulti-component material with superheated steam at a temperature ofabout 250° C. to about 400° C. and at a pressure up to about 86 atmwherein a caprolactam-containing vapor stream is formed.